Superheterodyne receivers are already known from the prior art. When the superheterodyne technique is used, the radio frequency signal to be received is mixed to a lower frequency by using one or more intermediate frequencies, which are essentially higher than the baseband. However, the heterodyne technique requires more complicated receivers, in which the manufacturing costs are higher than in the direct conversion technique, which is becoming more common today. In addition, power consumption is much higher in a heterodyne receiver than in a receiver implemented with the direct conversion technique. The direct conversion technique or zero intermediate frequency technique means using one frequency mixing for converting the modulated signal to be received into I/Q signals at a sufficiently low frequency, so that low pass filters can be used in channel filtering and that a separate new frequency mixing is not needed.
It would be possible to implement a direct conversion receiver by using a low intermediate frequency, the rate of which would be half of the channel spacing of the data transfer system. In that case, the mixer could be of the type that attenuates the image frequency, whereby the image frequency attenuation needed on the neighbouring channel (e.g. 26 dB) could be reached. To the best knowledge of the applicant, such a solution has not been disclosed in public.
FIG. 1 shows a block diagram of a known image frequency attenuating mixer for the reception of a frequency modulated signal. The signal to be received with an antenna 1 is filtered with a band-pass filter 2 and amplified with an amplifier 3. The signal of a local oscillator LO is phase-shifted 90° in block 4 and mixed with the upper branch of the amplified signal in a mixer 5. The mixing result is then phase-shifted 90° in block 6. The signal of the local oscillator LO is applied in the same phase (0° block 7) to a mixer 8, in which it is mixed with the lower branch of the amplified signal. The mixing result of the lower branch is then applied in the same phase (0° in block 9) to an adder 10, in which the signals formed as the mixing result are added together 10 into an intermediate frequency signal IF.
If the intermediate frequency is half of the channel spacing, it could be, for instance, 15 kHz. However, this is too low a frequency to enable an ordinary FM detector to operate as a demodulator with ordinary deviations, such as 8 kHz. In addition, the conventional LC detector based on a coil and a capacitor cannot be used, because the inductance value of the coil and the capacitance value of the capacitor should be very high. In addition, FM detectors that operate with the pulse counter principle are known, but even in such a detector, a low intermediate frequency causes a substantial distortion of the detected signal.